Method and apparatus for catching darts and other dropped objects

ABSTRACT

A dart or other dropped object having a rigid nose is dropped or launched down hole, typically during cementing operations, until it lands on a resilient seat within a dart catcher receptacle. By increasing a pressure differential across the landed dart to a predetermined amount, the dart extrudes through the seat and passes into a ported catcher sub. The seat member can be constructed of engineered composite material that does not yield in response to passage of the dart and returns to its original size following such passage.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 13/665,422, filed Oct. 31, 2012, currently pending, which claims priority of U.S. Provisional Patent Application Ser. No. 61/554,255, filed Nov. 1, 2011, all incorporated herein by reference.

STATEMENTS AS TO THE RIGHTS TO THE INVENTION MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

None

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a method and apparatus for performing cementing operations in oil and/or gas wells. More particularly, the present invention pertains to a method and apparatus for catching darts or other objects dropped or launched during such well cementing operations.

2. Brief Description of the Prior Art

Conventional rotary drilling rigs typically comprise a supportive rig floor, a substantially vertical derrick extending above said rig floor, and a traveling block, top drive unit or other hoisting mechanism that can be raised and lowered within said derrick. During drilling or servicing operations, such rig equipment is often used to manipulate tubular goods, such as drill pipe or drill collars, in and out of a well bore that extends into the earth's crust. Once a well has been drilled to a desired depth, large diameter pipe called casing is frequently installed in such well and, thereafter, cemented in place. The casing is typically installed to provide structural integrity to a well bore, and to keep geologic formations isolated from each other.

During casing installation operations, casing is typically inserted into a well in a number of separate sections of substantially equal length. As the bottom or distal end of the pipe string penetrates further into a well, additional sections of pipe are added to the ever-lengthening pipe string at the rig. After a sufficient length of casing has been installed into a bore hole, cement is typically pumped down the internal bore of said casing (or a smaller diameter workstring disposed within the casing), out the bottom of the casing, and up the annular space existing between the outer surface of the casing and the inner surface of the bore hole. When the cement hardens, a cement sheath is formed around the outer surface of the casing, anchoring the casing in place and isolating down-hole formations penetrated by said well from being in fluid communication with one another.

During cementing operations, darts, balls, plugs and/or other objects, typically constructed of rubber, plastic or other material, are frequently pumped into a well in connection with such cementing operations. In many instances, such items are suspended within a cementing head or other similar device at or near the surface until the objects are released or “launched” at desired points during the cement pumping process. Once released, such items join cement slurry flow and can be pumped down hole directly into a well. Typically, such darts, balls, plugs and/or other objects should be beneficially held in place within the slurry flow passing through a cement head prior to being launched or released without being damaged or washed away by such slurry flow.

Dart catchers, which are well known in the art, are devices that can be used to enhance cementing performance and subsequent drill-out operations. In most cases, at least one dart catcher is run downhole (frequently near the bottom of a casing string) via an inner workstring during installation of a casing string. While cement is being pumped into the inner bore of said workstring, a wiping dart or other droppable object can be launched and pumped downhole with the cement slurry until it “lands” within such a dart catcher.

After such wiping dart or other droppable object lands within said dart catcher, fluid flow through said dart catcher is restricted, resulting in an increase in pump pressure observable at the surface. Such pressure response provides a surface indication of the precise location of a cement wiping dart or other droppable object which, in turn, provides important fluid/cement displacement volume information regarding the cementing operation being performed.

After a wiping dart or other droppable object has been seated within a downhole dart catcher, many conventional dart catchers can then be shifted to a fluid by-pass position or otherwise opened to permit fluid flow through said dart catcher apparatus. Following such shifting or opening, pumping can resume and cement displacement operations can be completed. Typically, such wiping darts or other droppable objects will remain seated within in a dart catcher assembly, and can be retrieved upon removal of the workstring from the well.

Although such conventional dart catchers have a number of important benefits, existing dart catcher technology has certain important operational limitations. By way of example, such limitations relate to the methods of seating of wiping darts or other dropped objects, pressure indications related to the landing process, as well as flow capacity through such dart catchers after a dart has been landed. Thus, there is a need for an improved method and apparatus for catching darts or other dropped objects, particularly in connection with well cementing operations.

SUMMARY OF THE INVENTION

The present invention comprises a dart or other dropped object having a shearable nose member, as well as a dart catcher receptacle for receiving/landing said dart. For ease of reference, the droppable object is referred to herein primarily as a “dart” or wiper dart. However, it is to be understood that such terminology is not intended to be, nor should it be construed as, limiting in any manner. Accordingly, the droppable or launchable objects described herein are not limited to darts or other similar objects, and may include other droppable objects having different configurations.

The dart catcher receptacle of the present invention has an inner bore with a beneficial inner profile having a dart catching seat, described in more detail below that permits fluid flow through said catcher receptacle. Said dart catcher receptacle is positioned at a desired location downhole, typically prior to commencement of cementing operations. Thereafter, a dart of the present invention is launched or dropped and is pumped downhole.

Said dart proceeds downhole until it lands on the seat of the dart catcher receptacle. After said dart is landed within said dart catcher receptacle, the dart restricts fluid flow through the inner bore of said dart catcher receptacle. As fluid (typically cement slurry) is pumped down hole, a pressure indication, usually in the form of increased pump pressure, can be observed at the surface.

As cement slurry or other fluid is pumped into the well from the surface, a pressure differential is created across said dart. As said pressure differential increases to a predetermined amount, at least one sleeve on the outer diameter (“OD”) of the nose of said dart shears (giving a pressure indication at the surface) from said dart; the dart passes through said sleeve, but the sleeve remains within the inner bore of the dart catcher receptacle. In the preferred embodiment, shear screws or pins join said sleeve-like nose member to said dart, and shear at a predetermined differential pressure. Further, in the preferred embodiment, the shape of the nose sleeve is such that it leaves a smooth transition for the nose on a subsequently-launched dart to land and shear. The process can be repeated multiple times.

In the preferred embodiment, the wiper dart receptacle of the present invention has a restricted inner diameter to prevent darts from passing through said receptacle without the shearable nose member seating within said through-bore. However, the diameter of said through-bore is sufficiently large to permit passage of darts therethrough following shearing of the nose member; in most case, such darts pass through said through-bore and rest within a ported housing situated below the dart catcher receptacle.

The shearable sleeve nose member of the dart of the present invention includes an inner profile designed to ensure that wiper fins from darts are not cut or left behind by passing through the nose sleeve as they extrude into the catcher tube. Generally, the core or stem of a dart must be small enough to fit through the seat, yet strong enough not to break during use.

An alternative embodiment of the present invention comprises a dart catcher receptacle having a collet seat. Typically, collet seats are designed with collet finger seams oriented radially outward to permit said collet fingers to spread or expand outward when not restricted by an outer ring or other member. Such conventional collet seats can be problematic when pumping dart fins through spread collet fingers as dart fins can extrude through gaps formed between the collet fingers, resulting in said dart fins becoming stuck in said gaps or tearing away from the dart body.

In the preferred embodiment of the present invention, the collet fingers of the present invention are cut at an angle—that is, not radially outward from the central bore of the collet seat. As the collet fingers of the present invention spread or expand outward, said fingers push against each other but do not create gaps that can catch or tear dart fins.

As with other embodiments of the present invention, this embodiment of the present invention comprises a downhole dart catcher receptacle to catch drill pipe wiper darts. When such a dart is seated within said collet seat of the dart catcher receptacle, flow is restricted through said dart catcher receptacle. As a result, fluid (primarily pump) pressure from above creates a pressure differential across the seated dart, resulting in a pump pressure increase observable at the surface when each dart lands in the tool. The modular design of the collet and mating darts allows multiple darts to be utilized with a single tool.

Collet fingers of the present invention can beneficially flex, but always overlap one another to prevent any gaps between said fingers. The darts of the present invention have body sections small enough to fit through the expanded collet seat, yet strong enough not to break or bend during use. Dart passage pressure through said collet seat can be modified by adding/removing c-rings to the outer diameter of the collet seat, or by machining the collet fingers to have desired material characteristics.

In a second alternative embodiment of the present invention, portions of the downhole dart catcher receptacle are constructed of a resilient engineered composite material that is able to expand to pass a rigid dart nose, but subsequently contract back to its original size without permanently yielding. In this manner, multiple darts can pass through the inner bore of said downhole dart catcher receptacle with substantially the same pressure/force requirements.

In all embodiments, the dart catcher receptacle of the present invention provides sufficient clearance to allow a minimum equivalent flow diameter substantially the same as drill pipe (i.e., no flow restriction) and allow both forward and reverse flow through said tool. A catcher tube has enough bypass ports to allow minimum equivalent flow diameter as drill pipe (i.e., no flow restrictions) per foot and allow both forward and reverse flow through the tool. Flow slots are positioned such that reverse flow is possible even if a dart is pushed back up and covers the inlet to said catcher tube.

Dimensions described herein are for illustration only and are not to be construed as limiting in any manner. Further, as used herein, multiple references are made to darts. However, it is to be understood that the present invention is not limited solely to darts, and the present invention can beneficially function with other dropped objects including, without limitation, other configurations of wipers and the like.

BRIEF DESCRIPTION OF DRAWINGS/FIGURES

The foregoing summary, as well as any detailed description of the preferred embodiments, is better understood when read in conjunction with the drawings and figures contained herein. For the purpose of illustrating the invention, the drawings and figures show certain preferred embodiments. It is understood, however, that the invention is not limited to the specific methods and devices disclosed in such drawings or figures.

FIG. 1 depicts a perspective view of a first embodiment of a wiper dart equipped with the shearable sleeve member of the present invention.

FIG. 2 depicts a side sectional view of a dart catcher assembly of the present invention with a dart received within the inner seat profile of said dart catcher assembly.

FIG. 3 depicts a side sectional view of a dart catcher assembly of the present invention after a dart has passed through the inner seat profile of said dart catcher assembly.

FIG. 4 depicts a side sectional view of a dart catcher assembly of the present invention after multiple darts have passed through the inner seat profile of said dart catcher assembly.

FIG. 5 depicts a side sectional view of the detail area highlighted in FIG. 3.

FIG. 6 depicts a side sectional view of the detail area highlighted in FIG. 4.

FIG. 7 depicts a perspective view of a collet seat member of an alternative embodiment of the dart catcher of the present invention.

FIG. 8 depicts a bottom view of a collet seat member of an alternative embodiment of the dart catcher of the present invention.

FIG. 9 depicts a bottom view of a conventional prior art collet seat.

FIG. 10 depicts a side sectional view of a second alternative embodiment dart catcher assembly of the present invention.

FIG. 11 depicts a side sectional view of the detail area highlighted in FIG. 10.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 depicts a perspective view of a first embodiment of a wiper dart 100 equipped with the shearable sleeve 110 of the present invention. As depicted in FIG. 1, wiper dart 100 embodies a typical configuration of a conventional wiper dart, in that it comprises substantially cylindrical central stem member 101, as well as fin members 102 that extend radially outward from said central stem member 101. Said fin members 102 are flexible and can beneficially partially collapse or fold; in certain applications, said fin members 102 can be used to contact and wipe internal pipe surfaces, while being capable of passing through restrictions that may be encountered in such pipe.

Still referring to FIG. 1, wiper dart 100 comprises dart nose 104. Although the specific design of said nose 104 can vary, in the dart embodiment depicted in FIG. 1, said nose 104 includes tapered surface 103. Shearable sleeve 110 having tapered shoulder surface 111 is disposed on said nose 104. Said shearable sleeve 110 is secured to nose 104 using radial shear screws 120; it is to be observed that alternative shear devices such as, for example, shear rings or pins, can be used in place of said shear screws 120.

It is to be observed that the present invention is described with specific reference to wiper dart 100. However, it is to be understood that such reference is not intended to be, nor should it be construed as, limiting in any manner. Accordingly, the scope of the present invention extends to any number of different droppable or launchable objects, and is not limited merely to darts or other similar objects. Further, it is also to be observed that shearable sleeve 110 could be positioned at other locations on wiper dart 100, particularly if said wiper dart has a different shape or configuration.

FIG. 2 depicts a side sectional view of dart catcher assembly 10 of the present invention with wiper dart 100 received within an inner bore of said dart catcher assembly 10. Specifically, dart catcher assembly 10 of the present invention comprises axially connected receptacle assembly 20, housing assembly 30 and connection member 40.

Dart catcher receptacle assembly 20 comprises outer housing 21 having axial central through bore 22 and upper threaded connection member 23. Upper threaded connection 23 is adapted to connect to drill pipe, tubular workstring or other similar equipment. Seat member 24 has sleeve 25, which is disposed within said central through bore 22 of outer housing 21. Central through bore 26 extends through seat member 24 and defines an inner surface. It is to be observed that, when no obstruction (such as, for example, wiper dart 100) is present in through bore 26, fluid can flow through said dart catcher receptacle assembly 20.

Ported catch housing assembly 30 is connected to the bottom of dart catcher receptacle assembly 20. Said ported catch housing assembly 30 comprises substantially cylindrical body member 35 having axial central through bore 31 extending there through and defining a chamber within said body member. At least one port 32 extends through the side of cylindrical body member 35. Bowl-like seat 33 is disposed at or near the base of said ported catch housing assembly 30. Connection member 40 is connected below said ported sub 30 and has substantially cylindrical body section 43 and central through bore 45 extending there through, as well as upper connection member 41 and lower connection member 42.

In the preferred embodiment, dart catcher assembly 10 is connected to a tubular workstring and positioned at a desired location down hole, typically prior to commencement of cementing operations. Fluid (including, without limitation, cement slurry and/or displacement fluid) can be pumped down said tubular workstring. When desired, wiper dart 100 can be launched or dropped within said tubular workstring and is pumped down hole with cement slurry or displacement fluid. Said wiper dart 100 proceeds through such tubular workstring until it enters said dart catcher assembly 10 and lands within said dart catcher assembly 10 as depicted in FIG. 1 and as described in detail herein. After said wiper dart 100 is landed within said dart catcher assembly 10, dart 100 restricts fluid flow through bore 26 of seat member 24 (and said dart catcher assembly 10).

As cement slurry or other fluid is pumped into the well through said tubular workstring, a pressure differential is created across said dart 100. As the pressure above said dart 100 increases and said differential reaches a predetermined amount, axial forces acting on dart 100 cause shear screws 120 to shear, thereby releasing sleeve 110 from nose assembly 104 of dart 100. Downward pressure acting on dart 100 forces said dart 100 through said shearable sleeve 110.

FIG. 3 depicts a side sectional view of a dart catcher assembly 10 of the present invention after dart 100 has passed through shearable sleeve 110. As depicted in FIG. 3, said shearable sleeve 110 remains seated within through bore 26 of seat member 24. Dart 100 passes through said shearable sleeve 110, proceeds into ported catch housing assembly 30 and rests on seat 33. In most cases, the passage of dart 100 through the inner bore of shearable sleeve 110 and into catch housing assembly 30 results in a pressure indication observable at the surface (typically in the form of pump pressure reduction); after the obstruction caused by dart 100 is removed from seat member 24, a fluid restriction is removed and fluid can flow through said dart catcher assembly 10. Further, in the preferred embodiment, the shape of shearable sleeve 110 is beneficially designed to receive the nose of a subsequently-launched wiper dart, allowing such subsequent dart to land within said shearable sleeve 110.

FIG. 5 depicts a side sectional view of the detail area highlighted in FIG. 3. Shearable sleeve 110 has lower tapered shoulder 111 that rests on tapered landing shoulder 27 defined by the inner surface of through bore 26 of seat member 24. Because tapered landing shoulder 27 defines a smaller internal diameter than the largest outer diameter of shearable sleeve 110, said shearable sleeve 110 seats on, and cannot pass beyond, said landing shoulder 27. As discussed above, when pressure above a landed wiper dart (not shown) increases, shear screws 120 shear, allowing shearable sleeve 110 to release from dart 100. Downward pressure acting on dart 100 forces said dart through an inner bore of shearable sleeve 110 and into ported catch housing assembly 30 below; however, shearable sleeve 110 remains behind within through bore 26 of seat member 24.

FIG. 4 depicts a side sectional view of a dart catcher assembly 10 of the present invention after multiple wiper darts have passed through said dart catcher assembly 10 of the present invention. As depicted in FIG. 4, a first wiper dart can be dropped or launched and subsequently landed within dart catcher assembly 10. As downward fluid pressure is applied in accordance with the process described above, downward pressure acting on dart 100 forces said dart 100 through shearable sleeve 110 and into ported catch housing assembly 30 below, with shearable sleeve 110 remaining within through bore 26 of seat member 24.

Thereafter, a second wiper dart 150 can be dropped or launched and subsequently landed within shearable sleeve 110, which is itself landed within dart catcher assembly 10. As downward fluid pressure is applied in accordance with the process described above, downward pressure acting on dart 150 causes shearable sleeve 130 to separate from dart 150. Downward forces acting on said dart 150 force said dart 150 through stacked shearable sleeves 130 and 110, and into ported catch housing assembly 30. Shearable sleeve 130 remains seated within previously landed shearable sleeve 110; both stacked sleeves remain within through bore 26 of seat member 24.

FIG. 6 depicts a side sectional view of the detail area highlighted in FIG. 4. Shearable sleeve 130 has lower tapered shoulder 131 that rests within a central bore of shearable sleeve 110. Because the central bore of shearable sleeve 110 defines a smaller internal diameter than the largest outer diameter of shearable sleeve 130, said shearable sleeve 130 cannot pass beyond shearable sleeve 110, which in turn, cannot pass beyond landing shoulder 27 of seat member 24.

As discussed above, when pressure above a landed wiper dart (not shown) increases, shearable sleeve 130 is released from such dart. Downward pressure acting on such dart forces the dart through axially stacked shearable sleeves 130 and 110 and into ported catch housing assembly 30. The process can be repeated multiple times, with shearable sleeves, positioned in a stacked configuration, to accommodate multiple darts. The shearable sleeves of the present invention include inner profiles designed to ensure that wiper fins from darts are not cut or left behind when passing through said sleeves as darts/fins extrude into a ported catch housing assembly below.

A first alternative embodiment comprises a dart catcher receptacle assembly (such as dart catcher receptacle assembly 20 depicted in FIG. 20) having a collet seat member disposed within seat member 24. FIG. 7 depicts a perspective view of a collet seat member 200 of said first alternative embodiment of the dart catcher of the present invention. Said collet seat member 200 is mounted within such a dart catcher receptacle assembly so that the nose or leading tip of a dropped or launched dart will land within central bore 201 of said collet seat member. After such a wiper dart is landed within bore 201 of said collet seat member 200, said dart restricts fluid flow through said bore 201 of said dart catcher assembly.

As cement slurry or other fluid is pumped into the well through said tubular workstring, a pressure differential is created across said landed dart. As the pressure above said dart increases and said pressure differential reaches a predetermined amount, axial forces acting on dart cause collet fingers 202 of collet seat member 200 to expand or spread radially outward. Downward pressure acting on said dart forces said collet fingers 202 to continue to spread radially outward until said previously seated dart passes through central bore 201 of collet member 200.

FIG. 9 depicts a bottom view of a collet seat member 250 having conventional prior art collet fingers 252. Conventional collet fingers 252 are typically designed with collet finger seams 253 oriented radially outward to permit said collet fingers 252 to expand outward when not restricted by an outer ring or other member. However, the design of such conventional collet fingers 252 can be problematic when pumping dart fins through said collet fingers 252, as said dart fins can extrude through the gaps formed between the collet fingers, resulting in said dart fins becoming stuck in said gaps or tearing away from a dart body.

FIG. 8 depicts a bottom view of a collet member 200 of the present invention. As depicted in FIG. 8, seams 203 formed between collet fingers 202 of the present invention are oriented at an acute angle—that is, not radially outward from the central bore 201 of collet seat member 200. Said collet fingers 202 are still capable of expanding radially outward to permit passage of a wiper dart under application of predetermined forces; however, said collet fingers 202 ride against each other and do not create gaps or spaces along said seams 203 that could catch or tear dart fins. Put another way, collet fingers 202 of the present invention can beneficially flex and spread, but always overlap one another to prevent any gaps between said fingers.

When a wiper dart is seated within inner bore 201 of collet seat member 200 of the present invention, fluid flow is restricted through said bore 201. As a result, fluid (primarily pump) pressure from above said seated dart creates a pressure differential across the seated dart, resulting in a pump pressure increase observable at the surface when a dart lands in collet seat member 200. Moreover, the design of the alternative embodiment of the present invention utilizing collet seat member 200 allows multiple darts to be utilized with a single tool. Dart passage pressure can be modified by adding/removing c-rings to the outer diameter of the collet member 200 (such as in recessed section 204), or by machining or forming collet fingers 202 from material having desired physical properties or characteristics.

FIG. 10 depicts a side sectional view of a second alternative embodiment dart catcher receptacle of the present invention, while FIG. 11 depicts a side sectional view of the detail area highlighted in FIG. 10. In said second alternative embodiment of the present invention, seat 300 is mounted within downhole dart catcher receptacle assembly; in the preferred embodiment, said seat 300 is mounted within inner bore 26 of seat member 24. Said seat member 300 is constructed of an expanding engineered composite material that is able to expand to pass a rigid dart, but contract back to its original size without permanently yielding. In this manner, multiple darts can pass through the inner bore of said downhole dart catcher receptacle with substantially the same pressure/force requirement.

The body of the dart catcher receptacle of the present invention provides enough clearance to allow a minimum equivalent flow diameter as drill pipe or other tubular workstring (i.e., no flow restriction) and allow both forward and reverse flow through the tool. Catch housing assembly 30 has enough bypass ports 32 to allow minimum equivalent flow diameter as drill pipe (i.e., no flow restrictions) per foot and allow both forward and reverse flow through dart catcher assembly 10. Flow ports 32 can be elongate (slots) and positioned such that reverse flow is possible even if a dart within catch housing assembly 30 is pushed back upward and blocks the inlet to said catch housing assembly.

As used herein, multiple references are made to darts. However, it is to be understood that the present invention is not limited solely to darts, and the present invention can beneficially function with other dropped objects including, without limitation, other configurations of wipers and the like.

The above-described invention has a number of particular features that should preferably be employed in combination, although each is useful separately without departure from the scope of the invention. While the preferred embodiment of the present invention is shown and described herein, it will be understood that the invention may be embodied otherwise than herein specifically illustrated or described, and that certain changes in form and arrangement of parts and the specific manner of practicing the invention may be made within the underlying idea or principles of the invention. 

What is claimed:
 1. A dart catcher apparatus comprising: a) a dart having a rigid nose defining an outer diameter; and b) a housing having a central through bore, wherein a portion of said through bore has a reduced internal diameter defining a seat having an inner diameter smaller than said outer diameter of said nose, and wherein said seat comprises resilient material adapted to expand to allow passage of said rigid dart nose in response to a predetermined axial force acting on said dart.
 2. The dart catcher assembly of claim 1, wherein said seat is at least partially constructed of composite material.
 3. The dart catcher assembly of claim 1, wherein said seat does not permanently yield during passage of said dart.
 4. The dart catcher assembly of claim 1, wherein said seat returns to substantially its original size following passage of said dart.
 5. The dart catcher assembly of claim 1, further comprising a receptacle disposed below said seat adapted to receive a dart passing through said seat.
 6. The dart catcher assembly of claim 5, wherein said receptacle comprises a substantially cylindrical housing defining an inner chamber and at least one flow port extending from said inner chamber through said housing.
 7. The dart catcher assembly of claim 6, wherein said at least one flow port comprises an elongate aperture.
 8. A method for catching dropped objects in a well comprising: a) introducing a dart having a rigid nose defining an outer diameter into said well; b) receiving said dart on a seat in a dart catcher assembly, wherein said dart catcher assembly comprises a housing having a central through bore defining said seat, and wherein said seat has an inner diameter smaller than said outer diameter of said dart nose and is at least partially constructed of resilient material adapted to deform to allow passage of said rigid dart nose in response to a predetermined axial force; c) applying axial force to said dart; and d) passing said dart through said seat.
 9. The method of claim 8, wherein said seat returns to substantially its original size following passage of said dart.
 10. The method of claim 9, wherein said seat does not permanently yield in response to passage of said dart.
 11. The method of claim 9, wherein said dart catcher apparatus further comprises a receptacle disposed below said seat adapted to receive a dart passing through said seat.
 12. The method of claim 11, wherein said receptacle comprises a substantially cylindrical housing defining an inner chamber and at least one flow port extending from said inner chamber through said housing.
 13. The method of claim 12, wherein said at least one flow port comprises an elongate aperture. 